The last two decades have seen an explosion of interest in motion picture sound, in large part triggered by the ever-increasing quality of home high-fidelity systems. There is a continuing interest in new and improved motion picture soundtrack release formats.
Most films today are released with either conventional monophonic ("Academy") optical soundtracks or with stereo-variable-area (SVA) optical soundtracks with analog noise reduction, which are most widely known under the trademark "Dolby Stereo." "Dolby" and "Dolby Stereo" are trademarks of Dolby Laboratories Licensing Corporation.
The Academy format, originated in the 1930's, suffers from an extremely poor frequency response. Even though the soundtrack itself may extend to beyond 8 kHz, other elements within the recording and playback chain associated with the format restrict the bandwidth such that the audience will hear vary little above 4 or 5 kHz. In addition, the Academy format has relatively high distortion, a barely adequate signal-to-noise ratio and perhaps the greatest shortcoming of all: it is only monophonic (mono).
Dolby stereo is possibly the first motion picture optical soundtrack format which can truly be called high fidelity, and which is available to all theaters. Even though magnetic soundtracks (on 35 mm and 70 mm film) as long ago as the 1950's had moderately acceptable specifications, the high release print costs were in large part responsible for very few audiences ever hearing the magnetic versions. Stereo optical prints, on the other hand, have no premium cost above conventional optical soundtracks, and this has resulted in wide availability. A large majority of Dolby Stereo films are released "single inventory"--that is, a separate monophonic Academy print is not released because the producer considers the Dolby Stereo film to provided acceptable audible compatibility when played in Academy mono equipped theaters.
The Dolby Stereo optical soundtrack format provides four channels of information (left, center, right and surround) matrix encoded onto the two SVA optical film soundtracks. The original Dolby Stereo format employs Dolby A-type analog audio noise reduction. In the mid-1980's Dolby Laboratories introduced an improved analog audio processing system, Dolby SR, and that system has been applied to many Dolby Stereo films. The use of Dolby SR results in a dramatic improvement in volume range and frequency response. Even in a quiet and well-equipped theater, optical print noise is below the theater's ambient noise floor, while undistorted sound peaks are amply loud for most startling special effects. Frequency response extends to 16 kHz.
In spite of these advances in analog soundtrack fidelity, film soundtracks have long been considered a candidate for digital coding. Digital audio encoding has become integrated into the mainstream of both consumer and professional use. The Compact Disc ha earned wide consumer acceptance. As a result, a digital soundtrack would benefit from the popular conception that digital sound in inherently better, an undeniable added attraction at the theater box office.
In addition, a digital soundtrack may provide increased resistance to audible degradation of the soundtrack caused by the wear and tear of commercial exhibition, and can diminish the audible effects of projector wow and flutter. Multiple channels could be supplied on an optical soundtrack for both 35 mm and 70 mm print formats. The soundtrack's frequency and dynamic range specifications could exceed even that of current Dolby Stereo formats.
The recent announcement of two digitally-encoded optical soundtrack formats for 35 mm and 70 mm film, respectively, have reaffirmed the interest which exists in using digital soundtracks to improve motion picture sound in the theater. See "Digital Optical Sound on 35 mm Motion-Picture Film" by Syd Wiles et al, SMPTE Journal, November 1990, pp. 899-908 and "The Advent of Cinema Digital Sound" by Clyde McKinney, The Film Journal, August 1990, pp. 22 & 43 (a 70 mm system). Unfortunately, both of these formats locate the digital soundtrack information in the area formerly occupied by the analog soundtracks, making the these new digital formats incompatible with existing analog film formats and existing analog projection equipment.